Traditional power supply devices in motor vehicles have a generator, a battery, and various electrical consumers of the vehicle electrical system. The generator is driven by the internal combustion engine of the vehicle via suitable connecting means, e.g., the belt drive, and supplies the electricity required to charge the battery and supply power to the consumers. The output voltage of the generator is regulated to the desired level by an assigned voltage regulator and/or generator regulator, actual output voltage U1 of the generator optionally being varied within certain limits and adapted to preselectable requirements.
The generators customarily used are separately energized three-phase generators having an excitation winding through which an excitation current flows that is supplied by the battery and is switched by the generator regulator after activation of the ignition switch of the vehicle (“ignition on”). In order for a generator to be able to “start” at all, it needs the excitation current from the battery in the start phase. The excitation current creates a magnetic field in the rotor of the generator, thereby inducing a voltage in the stator winding of the generator when the rotor is rotating.
Since vehicle electrical systems need different voltages to supply the various electrical consumers, motor vehicles are known to use power supply devices that include multiple voltage systems at different voltages. Such an automotive power supply device is described in German Patent Application No. DE 38 12 577, for example, and includes two generators supplying voltages U1 and U2 to charge storage devices, i.e., batteries, and the assigned electrical consumers of the vehicle electrical system. Each generator is assigned its own battery. There is no provision for complete decoupling of the individual voltage systems.
Another power supply for a vehicle electrical system using two generators is known from German Patent Application No. DE 101 06 723, in which a PWM inverter is provided for each generator designed as a three-phase generator having an excitation winding and stator windings, a connection to one battery at a time being establishable via this PWM inverter. The generators supply different output voltages via which two partial vehicle electrical systems are supplied with power. No details are given about the start phase of the generators.